how big is your BP?
you don't need to buy anything right away, you can use something like a cool whip container upside-down with a door cut into it (melt or sand any sharp edges)

I have used plastic saucers from plant pots, which they just curl up underneath and it presses down on them and makes them feel snug (use the plastic kind, not the ceramic kind)

As for a good pet store to go to if you want, how close are you to Plaistow NH?